1. Field of the Invention
Aspects of the present invention relate to a multiblock copolymer including a polysulfone repeating unit, a sulfonated polysulfone repeating unit, and a polydialkylsiloxane repeating unit, a method of preparing the multiblock copolymer, a polymer electrolyte membrane prepared from the multiblock copolymer, a method of preparing the polymer electrolyte membrane, and a fuel cell employing the polymer electrolyte membrane. In particular, aspects of the present invention relate to a multiblock copolymer that has a high ionic conductivity, high hydrophobicity, and good mechanical properties and that has various structures to increase its selectivity to a solvent used in a polymer electrolyte membrane. Aspects of the present invention also relate to a method of preparing the multiblock copolymer, a polymer electrolyte membrane prepared from the multiblock copolymer, a method of preparing the polymer electrolyte membrane, and a fuel cell including the polymer electrolyte membrane.
2. Description of the Related Art
Fuel cells can be classified into polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), etc., according to the type of electrolyte used in the fuel cell. The working temperature and constituent materials of fuel cells vary according to the type of electrolyte used in a cell.
According to how fuel is supplied to the anode, fuel cells can be classified into an external reformer type, where fuel is supplied to the anode after being converted into a hydrogen-rich gas by an external reformer, and an internal reformer type or direct fuel supply type where fuel in a gaseous or liquid state is directly supplied to the anode.
A representative example of a direct liquid fuel cell is a direct methanol fuel cell (DMFC). DMFCs use an aqueous methanol solution as fuel and a proton exchange polymer membrane with ionic conductivity as an electrolyte. (Therefore, a DMFC may also be a PEMFC) DMFCs are small and lightweight, but can achieve a high output density. In addition, an energy generating system having a simpler structure can be manufactured using PEMFCs.
A basic structure of a PEMFC includes an anode (fuel electrode), a cathode (oxidant electrode), and a polymer electrolyte membrane disposed between the anode and the cathode. A catalyst layer for facilitating the oxidation of fuel is formed on the anode of the PEMFC, and a catalyst layer for facilitating the reduction of an oxidant is formed on the cathode of the PEMFC.
In the anode of the PEMFC, proton ions and electrons are generated as a result of the oxidation of fuel. The proton ions migrate to the cathode through the polymer electrolyte membrane, and the electrons migrate to an external circuit (load) through a wire (or a current collector). In the cathode of the PEMFC, the proton ions transmitted through the polymer electrolyte membrane and the electrons transmitted from the external circuit through a wire (or a current collector) combine with oxygen, thereby generating water. The migration of electrons via the anode, external circuit, and cathode produces an electric current.
In PEMFCs, the polymer electrolyte membrane acts as an ionic conductor enabling the migration of proton ions from the anode to the cathode and as a separator preventing a mechanical contact between the anode and the cathode. Thus, a high ionic conductivity, high electrochemical stability, a high mechanical strength, high thermal stability at working temperature, easy processibility into a thin film, etc., are required for the polymer electrolyte membrane.
Currently available materials for the polymer electrolyte membrane include polymer electrolytes, such as a perfluorinated sulfonate polymer (for example, NAFION®, which is a registered trade mark of Dupont) having a fluorinated alkylene backbone and fluorinated vinylether side chains having sulfonic acid groups at the terminals thereof. Polymer electrolyte membranes composed of such a polymer electrolyte contain an appropriate amount of water and exhibit high ionic conductivity.
However, the crossover of methanol may be high in such an electrolyte membrane, and the manufacturing cost of the electrolyte membrane is high. Furthermore, the ionic conductivity of the electrolyte membrane decreases significantly at a working temperature of 100° C. or higher due to the loss of water caused by evaporation, and eventually the electrolyte membrane loses its inherent function. Thus, it is almost impossible to operate PEMFCs using such a polymer electrolyte membrane at 100° C. or higher under atmospheric pressure. For this reason, conventional PEMFCs have been operated at temperatures less than 100° C., such as, for example, at a temperature of about 80° C.
In addition, as the ionic conductivity of an electrolyte membrane increases, the water transmission of the electrolyte membrane also increases. However, the increase in water transmission leads to an increase in methanol transmittance. Thus, the requirements for both high ionic conductivity and low methanol transmission cannot be simultaneously satisfied. In other words, in an electrolyte membrane that can pass a predetermined concentration of methanol solution, when the relative amount of water in the methanol solution that can pass the electrolyte membrane with respect to a standard electrolyte membrane (for example, Nafion 115) is 1 or greater, and the relative amount of methanol in the methanol solution that can pass the electrolyte membrane is 1 or less, the electrolyte membrane is useful as a DMFC electrolyte membrane.
To overcome the above-described problems, research into polymer electrolyte membranes as replacements for the NAFION electrolyte membrane is being intensively conducted. As a material for such polymer electrolyte membranes, a block copolymer comprising hydrocarbon repeating units, such as styrene repeating units, ethylene-r-butylene repeating units, isobutylene repeating units, etc., is known.
However, such a block copolymer leads to methanol crossover and serious swelling of the electrolyte membrane, and thus the dimension stability of the membrane and electrolyte assembly (MEA) is poor. In addition, the hydrophobic properties and the mechanical properties of the electrolyte membrane are not so good.